Many alloys of aluminum and other elements may be heat treated to control and improve their mechanical and physical properties. In one such process, the basic alloy material of the desired composition is melted and cast into a desired shape. The cast alloy material is deformation processed by rolling, extrusion, drawing, machining, or other technique to a desired size and shape. The deformation processed alloy material is thereafter heat treated to achieve particular mechanical and physical properties.
The most common heat treatment for aluminum alloys is solution treating and aging. The aluminum alloy is heated to a temperature at which the alloying elements dissolve into solid solution in the matrix. The aluminum alloy is then rapidly cooled ("quenched") to retain the alloying elements as solute in solid solution. The material is thereafter heated to an intermediate temperature ("aged") at which second-phase particles of different types, generally termed "precipitates", form by diffusional reactions between the alloying elements and possibly the aluminum matrix material. There may also be an associated mechanical deformation of the material to alter the grain structure and precipitate character of the material.
The presence of these precipitates improves the strength properties of the material by various mechanisms, but they may also have adverse effects on other properties such as elongation, fracture toughness, corrosion, stress corrosion, weldability, etc. The nature of the properties achieved depend upon the composition of the aluminum alloy. At the present state of metallurgical understanding, it is not possible to predict with certainty the response of a particular aluminum alloy to solution heat treating and aging procedures. Care is taken to develop particular solution treating and aging procedures for each alloy family. Thus, because a particular heat treatment is successful for one alloy family does not at all suggest that it will be successful for another alloy family, because of the different aging responses of the different materials.
Alloys of aluminum (Al) and lithium (Li) are of interest for aerospace applications because lithium has a low atomic weight and can serve, alone or in combination with other alloying elements, as the basis for solution treating and aging procedures. In one recently developed alloy, copper (Cu), scandium (Sc), and other alloying elements are added to an Al-Li material. This Al-Cu-Li-Sc alloy shows promise for a number of applications, such as cryogenic tankage and other structural elements. The addition of scandium has been found to markedly alter the alloy chemistry and grain structure.
When the Al-Cu-Li-Sc alloy is heat treated by solution treatment followed by a conventional aging at 130.degree. C. for 8-12 hours to achieve high yield and tensile strengths, the elongation to failure is relative low. The result is that the fracture toughness is lower than desired for applications such as cryogenic tankage. There is therefore a need for an improved approach to the heat treatment of alloys, which allows the alloy to be tailored to exhibit specific combinations of mechanical and physical properties. The present invention fulfills this need, and further provides additional advantages as well.